1. Field of the Invention
The present invention relates to carrier-injection organic electroluminescence (EL).
2. Description of the Related Art
Products to which a carrier-injection organic EL light-emitting phenomenon is applied are being put to practical use (for example, see Patent Document 1). There are generally two excited states a light-emitting material can be in, which are a singlet excited state and a triplet excited state, and the former is considered to be probabilistically about one-third of the latter.
The ground state of a light-emitting material is a singlet state, and for ordinary organic molecules, the transition from the singlet excited state to the ground state is possible, but the transition from the triplet excited state to the ground state is forbidden. In other words, although light emission can be obtained through the transition from the singlet excited state to the ground state, in many cases, transition from the triplet excited state to the ground state accompanied by light emission does not occur, and the transition from the triplet excited state to the ground state is made via various levels. In that process, energy is released as heat. Such a phenomenon is called thermal deactivation.
As described above, the probability of being in the singlet excited state is one-third of that of being in the triplet excited state. Therefore, much of energy given to the light-emitting material is lost as heat, which results in poor efficiency. In addition, a large amount of heat is generated, which causes deterioration of an organic material.
In order to solve such problems, a technique has been developed for increasing light emission efficiency by adding an organic metal complex containing a heavy metal (particularly, iridium) to a light-emitting material at about 5 wt % (see Non-patent Document 1). In such a technique, a main constituent is called host, and an organic light-emitting material added is called guest (or dopant). In that technique, as shown in FIG. 2A, a host 202 in a light-emitting layer is evenly doped with organic light-emitting material molecules 201.
Note that when the concentration of an organic light-emitting material is higher than or equal to 10 wt %, light emission from one organic light-emitting material molecule is absorbed by the other organic light-emitting material molecules, which results in a decrease in light emission efficiency (this is called concentration quenching), and thus it is necessary to avoid the concentration of an organic light-emitting material of higher than or equal to 10 wt %.
In this technique, a host molecule in the triplet excited state returns to the ground state by transferring the state to an organic light-emitting material molecule, and conversely the organic light-emitting material molecule receives the excited state from the host molecule and is brought into the triplet excited state. The organic light-emitting material molecule can transit to the ground state from the triplet excited state accompanied by light emission owing to spin-orbit interaction.